1. Field of the Invention
The present invention relates to a gas discharge tube and a method for forming an electron emission film into the gas discharge tube, and more particularly to a thin gas discharge tube having a diameter of approximately 0.5 to 5 mm and a method for forming an electron emission film into the gas discharge tube suitably adapted for such a gas discharge tube.
2. Description of the Related Arts
A conventional gas discharge tube is provided with electrodes at end edges with respect to the longitudinal direction of the discharge tube for extending a discharge in the longitudinal direction. Films of an electron emission material (electron emission film) that improves discharge characteristics are directly formed on filaments serving as the electrodes. Therefore, the filaments to which the electron emission film is vapor-deposited are adhered to be fixed onto the end edges of the discharge tube in manufacturing the gas discharge tube.
There are other gas discharge tubes than the above-mentioned discharge tube, one of which is provided with a lot of electrodes on the side of the tube. A display device in which a plurality of such slender, long gas discharge tubes are arranged has been known.
A screen of this display device is composed of a great number of light emitting elements (tubular-light-emitting elements: gas discharge tubes) arranged in a line direction (a column direction) of the screen. The light emitting elements are formed of hollow, slender, long glass tubes having a diameter of approximately 0.5 to 5 mm whose outer walls are provided with electrodes and in which a discharge gas is enveloped. Known display devices of the above-mentioned type are a large gas discharge display panel disclosed in Japanese Unexamined Patent Publication No. Sho 61(1986)-103187 and an image display device disclosed in Japanese Unexamined Patent Publication No. Hei 11(1999)-162358. The display device of this type has advantages of reduced assembling man-hour, light-weight and low cost, easy to change the screen size or the like.
A gas discharge tube used for this display device has a structure of having a plurality of electrodes that can generate facing discharge or surface discharge inside the discharge tube. A discharge is generated in a direction between the side faces of the discharge tube so as to obtain a great number of light-emitting points in one tube.
Considering withstand voltage of a driving circuit and a cost of circuit components, voltage for generating a discharge between electrodes (voltage for initiating a discharge) in the gas discharge tube is desired to be low. Accordingly, an electron emission film is formed onto a discharge face for improving discharge characteristics.
This gas discharge tube is provided with electrodes at the outer wall of the tube as described above, so that the formation of the electrodes is easy, but the electron emission film is not attributed to the improvement of the discharge characteristics since the electron emission film is not in direct contact with the discharge gas even though the electron emission film is directly formed onto the electrodes.
In order to solve this problem, the electron emission film may be formed onto the inner wall of the tube, not on the electrodes positioned outside of the discharge tube. This can improve the discharge characteristics.
However, it is extremely difficult to form the electron emission film on the inner wall of the slender, long glass tube having a diameter of 2 mm or less and a length of 200 mm or more.
For example, a film-formation by a vapor deposition method brings a non-uniform distribution of the film thickness in the tube since evaporating molecules of a material for forming an electron emission film introduced from the tube edge is significantly accumulated in the vicinity of the tube edge. The uneven thickness of the electron emission film causes variations in the voltage for initiating the discharge at a great many of light-emitting points in the tube, thereby arising a problem of narrowing the margin of the light-emitting operation.